液體介面之電操控：電濕潤、介電濕潤及反濕潤

Abstract

近十年來，液體介面的電操控已廣泛運用在微流體元件的應用上。然而，液體於元件內之動態運動行為無法從實驗中清楚解析。因此，我們開發一個考慮弱介電質模型的電流體力學理論與相場法之間耦合的數值模擬方法來求解液體介面受到外加電場影響後隨時間變形的課題。透過此弱介電質模型的假設，我們提出的數值模擬模型可以模擬無論以導電液體或介電液體為操控目標的微流體原件。 我們針對電濕潤顯示器畫素內導電液體與介電液體之介面的動態運動行為進行討論。我們所發展的數值模型成功模擬了畫素開啟時的反濕潤與電濕潤現象及畫素關閉時的流體攤平現象。考慮液體接觸角隨電壓的變化於模型中，電壓與開口率的模擬結果與相關的實驗測試相當吻合。此外，我們亦藉由數值模擬模型的參數分析探討了電濕潤顯示器在驅動過程中所發生的油墨分裂、油墨跳墨以及油墨平鋪不均等元件缺陷的現象。釐清元件缺陷的原因後，我們成功開發了一個高穩定的電濕潤顯示器。 此篇論文亦由宏觀與微觀的層面討論了介電液體與空氣間介面受到外加電場影響所產生的介電濕潤現象。我們藉由實驗與數值模型探討了液體表面的皺褶行為、電極形狀與液滴大小對液體等向流動之課題，以及液體在電壓釋放後之反濕潤現象。這些結果提供了許多與介電濕潤理論相關的資訊於介電濕潤光閘元件的開發上。 過去已提出的液體介面電操控的方式僅能製作出週期性之形狀(對稱波)或球狀結構(僅有兩個主曲率半徑)。在此研究中，我們開發了兩種能夠建構更複雜流體介面形狀的電操控方法。第一種電操控方式是藉由電容值感應的回饋控制使得每個控制電極上方之流體厚度能夠被準確掌控；而第二種電操控方式是透過電極產生多個週期性的輸出波形導致流體以傅立葉組合的形式建構出複雜的形狀。過去在電濕潤應用所使用的材料可以直接運用於此開發元件上。此外，因為流體在操控時的反應速度足夠慢(毫秒-微秒)，使得傳統的電子控制元件(微秒-奈秒)足夠匹配。最後，由於此元件中的導電液體不會碰觸電極上方的固態表面，因此介電材料的特性退化議題可以被忽略。

Electrically based control of the geometry or transport of a fluid meniscus has been a technological trend used in microfluidic applications in the last decade. Aside from experiments, numerical models could provide an effective tool for testing the feasibility of device applications. Here, we present a numerical simulation technique to calculate the deformation of fluids at interfaces by coupling the electrohydrodynamic (EHD) theory considering the assumption of leaky dielectric model with the Phase Field Method (PFM). With the assumption, the proposed model could simulate the interaction of a fluid-fluid interface with an electric field, using conductive liquids as well as dielectric liquids. The fluid dynamic behavior within a pixel of an electrowetting display (EWD) is studied. The complete switch processes, including the break-up and the electrowetting stages in the switch-on process (with voltage) and the oil spreading in the switch-off process (without voltage), are successfully simulated. By considering the factor of the change in the apparent contact angle at the contact line, the electro-optic performance obtained from the simulation is found to agree well with its corresponding experiment. In addition, the proposed modeling is used to parametrically predict the effect of interfacial (e.g., contact angle of grid) and geometric (e.g., oil thickness and pixel size) properties on the defects of an EWD, such as oil dewetting patterns, oil overflow, and oil non-recovery. With the help of the defect analysis, a highly stable EWD is both experimentally realized and numerically analyzed. Dielectrowetting effects of surface wrinkling, droplet size, isotropic vs. anisotropic spreading, electrode geometry, and deterministic dewetting, are presented both experimentally and by the developed model. The dynamic behavior of the two phase system has been accurately characterized on both the macro and microscopic level. These results can be used to further optimize dielectrowetting optical shutter and to provide a deeper theoretical insight into the operating physics of dielectrowetting effect. Existing techniques for electronic control of the interface between two immiscible fluids are typically limited to simple periodic geometries (symmetric waves) or spherical geometries (only two principle radii of curvature). Presented here, is a new technique with much more sophisticated electronic control of fluid meniscus geometry. Two distinct approaches are demonstrated: (1) application of voltages, electrical capacitance sensing of meniscus geometry, followed by further feedback control of the applied voltages based on the sensed electrical capacitance; (2) use of multiple periodic voltage waveforms and wave propagation across the meniscus to build up complex meniscus geometries by Fourier construction. The results can be achieved using conventional materials, and the fluids respond with speeds that are adequately slow (ms-µs) such that even conventional control electronics (µs-ns) are more than adequate. Furthermore, because the conducting fluid never dewets the oil film from the solid surface, dielectric degradation issues are likely eliminated.